All sins "miss the mark" of what we should do, but some miss the mark more than others. We don't classify them nowadays, not where I've been anyway, but it is true that some sins are worse than others, something even Jesus alluded to.

FWIW, without going into all the details invlolved in the RC distinction between venial/mortal sins, my confessor does not require me to confess everytime before I receive communion, but there are certain sins that would render me unprepared for the reception of Communion if not confessed.

EDIT: I just noticed where this was posted. I'm not OO so I apologize if my reply ends up being inconsistent with OO replies here, but I do not expect this to be an area of difference between us, so I will leave it up with this disclaimer.